Manually assigning mirror pairs?
Yes bones are mirrored if you enable x-axis mirror and as long as everything in the name is the same except 'Left' and 'Right' and can even be directly next to a letter if you use it as suffix. If you use 'Left' and 'Right' as prefix, then separate with '.' (period) or '_' (underscore)

But if you use 'L' and 'R' it cannot be next to a letter, but has to be separated with '.' (period) or '_' (underscore)

For example prefixes:


L.Bone and R.Bone
Left.Bone and Right.Bone
L_Bone and R_Bone
Left_Bone and Right_Bone


or suffixes:


Bone.L and Bone.R
Bone.Left and Bone.Right
Bone_L and Bone_R
Bone_Left and Bone_Right
BoneLeft and BoneRight